At approximately day 1

Uterus begins to develop.

Reference

O’Rahilly R, Muller F. 2001. Human embryology & teratology. Third edition. New York: Wiley-Liss.

At approximately day 1

Fine mammary capillaries start to form.

Duration: 1 week

Reference [PubMed Link]

Laurence DJ, Monaghan P, Gusterson, BA. 1991. The development of the normal human breast. Oxford Reviews of Reproductive Biology 13:149-174.